Evaluate Reverse Polish Notation

Description

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

• The valid operators are '+', '-', '*', and '/'.
• Each operand may be an integer or another expression.
• The division between two integers always truncates toward zero.
• There will not be any division by zero.
• The input represents a valid arithmetic expression in a reverse polish notation.
• The answer and all the intermediate calculations can be represented in a 32-bit integer.

Example 1:

Input: tokens = [“2”,“1”,”+”,“3”,”*“] Output: 9 Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: tokens = [“4”,“13”,“5”,”/”,”+“] Output: 6 Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: tokens = [“10”,“6”,“9”,“3”,”+”,“-11”,”*”,”/”,”*”,“17”,”+”,“5”,”+“] Output: 22 Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5 = ((10 * (6 / (12 * -11))) + 17) + 5 = ((10 * (6 / -132)) + 17) + 5 = ((10 * 0) + 17) + 5 = (0 + 17) + 5 = 17 + 5 = 22

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

Code

Stack

Time Complexity: $O(N)$, Space Complexity: $O(N)$

要注意是 op2 += op1，因為從 stack pop 下來順序和原本 strings 是相反的。

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> st;
        for(auto& token: tokens) {
            if((token == "+") || (token == "-") || (token == "*") || (token == "/")) {
                int op1 = st.top(); st.pop();
                int op2 = st.top(); st.pop();
                if(token == "+") op2 += op1;
                if(token == "-") op2 -= op1;
                if(token == "*") op2 *= op1;
                if(token == "/") op2 /= op1;
                st.push(op2);
            } else {
                st.push(stoi(token));
            }
        }
        return st.top();
    }
};

Fancy C++ lambda expression solution (using stack)

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        unordered_map<string, function<int (int, int) > > map = {
            { "+" , [] (int a, int b) { return a + b; } },
            { "-" , [] (int a, int b) { return a - b; } },
            { "*" , [] (int a, int b) { return a * b; } },
            { "/" , [] (int a, int b) { return a / b; } }
        };
        std::stack<int> stack;
        for (string& s : tokens) {
            if (!map.count(s)) {
                stack.push(stoi(s));
            } else {
                int op1 = stack.top();
                stack.pop();
                int op2 = stack.top();
                stack.pop();
                stack.push(map[s](op2, op1));
            }
        }
        return stack.top();
    }
};

Source

• Evaluate Reverse Polish Notation - LeetCode